Most of the time people fall off when they "quit riding". At some point, in a buck or a spin or a sudden sidestep, you lose your balance or seat, and you get to this place where you decide that you aren't going to be able to stay on. Sometimes you really have no choice, (like the saddle has slid over, or you just got too far off to the side) but there are other times when you , yourself "stop riding" and that's when you go off. You kind of resign yourself to falling and you stop doing all the things that you do to stay on a horse, like sit up, keep your heels down and such. If you keep riding, you keep yourself mentally in the saddle, mentally present and menatlly planning for the moment when the horse stops bucking or rearing. It's very much a mental thing, but there are lots of times that a rider could have stayed on if they had mentally not quit riding. Myself included.